FUEL TANK STRUCTURE

A fuel tank structure that makes it difficult for mud, water, dust or the like to enter a fuel tank and that can restrict a fuel leakage path when a vehicle falls. A fuel cap includes a cap body mounted to a fuel supply port of a fuel tank, and a pressure adjusting device provided in the cap body to adjust a pressure in the fuel tank. In the fuel cap, a space is formed as a hermetically closed space in which the pressure adjusting device is accommodated, and the space communicates with an exterior of the fuel tank via a breather tube.